Il 22/04/2013 14:50, Luiz Capitulino ha scritto: > On Mon, 22 Apr 2013 11:37:15 +0200 > Paolo Bonzini <pbonz...@redhat.com> wrote: > >> Il 22/04/2013 08:55, Gerd Hoffmann ha scritto: >>>>>>> Question for the libvirt guys: Is it ok for libvirt to just extend the >>>>>>> existing screendump command? Can libvirt figure there is a new >>>>>>> (optional) parameter? See patch #5. >>>>> >>>>> Nope, QMP can't do that. I argued for such capabilities, but the >>>>> "create a new command" philosophy prevailed. >>>>> >>>>> Go forth and multiply commands! And have fun picking command names that >>>>> aren't fugly. >>> Oh joy. Lets just enumerate things & use "screendump2" ... >> >> QMP can't do that _yet_. >> >> Let's fix it instead... > > The point is that we have chosen not to do so a while ago. In a nutshell, > Anthony thinks that we should have the same compatibility contract of > a C API.
We've been adding fields to types since 0.15, sometimes in the middle of a struct (since 1.2). If the C API is a requirement, it should also be a requirement for structs. But there are plenty of ways (some nicer, some uglier) to have different API versions in C. For example, I think the QIDL patch had ways to annotate each parameter independently. You can annotate each argument with the "first version this appeared in" and complicate the C API generator to generate multiple C functions for the same command. It is then the downstream's responsibility not to backport extra arguments without a full-blown rebase to a newer version (the same as for C libraries). Paolo
